1. Field of the Invention
The present invention relates to an improvement of a magneto-optic recording medium capable of being written/read by irradiating a laser beam thereon.
2. Description of the Related Art
In a recording method for recording information on a recording medium where information marks (magnetic domain) are read out by utilizing a magneto-optic interaction, the information is recorded on the recording medium as binary information marks in such a manner that first, the recording medium having such a magnetic thin layer as a perpendicular magnetization thin layer is initialized so that a magnetization orientation of the magnetic thin layer is preliminarily aligned in one direction perpendicular to a surface of the magnetic thin layer. Then, magnetic domains corresponding to the information marks are formed to have an magnetization orientation in a reverse direction of the initial magnetization by spot-heating such as irradiation of a laser beam spot with applying the external magnetic field.
In the recording method, before rewriting the information preliminarily recorded on the recording medium, it requires a considerable time to erase (or initialize) the information recorded. As a result, there is a problem that it is impossible to realize a recording of information at a high data transfer rate.
As a countermeasure, in order to save the time of initialization, various kinds of overwriting methods without erasing information are proposed or put into practice.
Among the overwriting methods, there is one so called the light intensity modulation direct overwriting technic, which seems promising as a high density recording technic combined with a magnetic super-resolution technic in future.
As a basic technic of a recording medium to realize this light intensity modulation direct overwriting, there is one proposed in the Japanese Patent Laid-open Publication 62-175948/87.
In the magneto-optic recording method disclosed therein, there is employed a recording medium having a laminated structure of first and second magnetic thin films of rare earth-transition metal. The outline of operation of the magnetic recording is as follows.
A first elevated temperature state by heating the first magnetic thin film up to a temperature T1 higher than Curie temperature Tc of the first magnetic thin film under a first external magnetic field without causing inversion of the sublattice magnetization of the second magnetic thin film; and a second elevated temperature state by heating the second magnetic thin film up to a temperature T2 higher than the temperature T1 and high enough to invert the sublattice magnetization of the second magnetic thin film under the first external magnetic field, which two temperature states are respectively conditioned corresponding to the digital information "0" or "1" to be recorded.
In cooling process, recorded marks (magnetic domains) corresponding to the digital information "0" and "1" are formed on the first magnetic thin layer corresponding to the magnetization orientation of the sublattice of the second magnetic thin layer based on the exchange bonding force between the 1st and 2nd thin layers due to exchange interaction. Then, only the magnetization orientation of the sublattice of the second magnetic thin layer is inverted in one direction at room temperature by applying a second external magnetic field. Thereby, the direct overwrite can be realized.
FIG. 1 is a schematic view for explaining the magnetization states of 1st and 2nd magnetic layers in response to a change of temperature from room temperature to recording temperature upon the recording operation or the rewriting operation.
Next, the description is given of the detailed operation of the magneto-optic recording method in the prior art referred to FIG. 1.
In FIG. 1, a magneto-optic disc (referred to as a "disc") comprises at least a 1st magnetic thin layer as a recording layer, which exhibits high coercivity at room temperature and has low reverse magnetization temperature, and a second magnetic thin layer as a reference layer, which exhibits low coercivity at room temperature and has higher reverse magnetization temperature. Both the layers have perpendicular magnetization. The disc is assumed to be rotated in a certain direction and a laser beam (not shown) is irradiated to provide heat to the 1st and 2nd layers of the disc so as to raise the temperature T1 or T2 corresponding to a low level of digital information or a high level thereof. Further, upon a recording or rewriting operation, an initial external magnetic field Hini and an external recording magnetic field Hex are always applied to the disc in directions as shown with arrows. Thus, the external recording magnetic field Hex is applied to the 1st and 2nd magnetic layers heated at the low temperature T1 or at the high temperature T2 by the laser beam during a rotation of the disc. Thereafter, the initial external magnetic field Hini is applied to the 1st and 2nd magnetic layer cooled down at room temperature during the rotation of the disc.
As initial states, there are two cases, a state A and a state B.
In the state A as the initial state, the 1st magnetic layers is assumed to be magnetized in a direction shown with an arrow "a" in response to a digital signal of a low level "0". The 2nd magnetic layer is magnetized in a direction shown with an arrow "a" as mentioned hereinafter.
In the state B as another initial state, the 1st magnetic layer is magnetized in a direction shown with an arrow "b" in response to a digital signal of a high level "1", and the 2nd magnetic layer is magnetized in the direction "a". It should be noted that the 2nd magnetic layer is always magnetized in the direction shown with the arrow "a" after passing through the initial external magnetic field Hini in the direction of "a".
As an elevated temperature state, there are two cases, a state C corresponding to the lower temperature T1 and a state D corresponding to the higher temperature T2.
In the state C, the magnetization of the 1st magnetic layer vanishes at the lower temperature T1 higher than Curie temperature Tc1 of the 1st magnetic layer by being irradiated with the laser beam, but the 2nd magnetic layer remains in the initial magnetization even when the external recording magnetic field Hex shown with an arrow "b" is applied to the 2nd magnetic layer.
In the state D, the magnetization of the 1st magnetic layer vanishes at the higher temperature T2 which is higher than the lower temperature T1 by being irradiated with the laser beam, and the magnetization orientation of the 2nd magnetic layer is changed to a direction shown with the arrow "b" due to the external recording magnetic field Hex in the direction "b".
As interim states during the rotation of the disc, there are two cases, a state A1 which is changed from the state C and a state E which is changed from the state D. These states A1 and E are realized to be cooled down below Curie temperature Tc1 during the rotation of the disc.
In the state E, the 1st magnetic layer is magnetized in the direction "b" subjected by a magnetic field ("b") of the 2nd magnetic layer in a temperature range lower than the Curie temperature Tc1 of the 1st magnetic layer as the disc is rotated to be cooled down.
Then, as the disc further rotates, the state E is further changed into the state B under the initial magnetic field Hini applied to the 1st and 2nd magnetic layers in the direction "a" at the room temperature, wherein the magnetization orientation of the 2nd magnetic layer is changed corresponding to the direction "a" of the initial magnetic field Hini though the magnetization orientation "b" of the 1st magnetic layer remains as it is.
In the state A1, the 1st magnetic layer is magnetized in the direction "a" subjected by a magnetic field ("a") of the 2nd magnetic layer in a temperature range lower than the Curie temperature Tc1 of the 1st magnetic layer as the disc is rotated to be cooled down.
Then, as the disc further rotates, the initial magnetic field Hini is also applied thereto, however, the magnetization orientation thereof is the same as that of the 2nd magnetic layer. Thus, the state A is the same as the state A1.
As well known, when the 1st and 2nd magnetic layers are facing to each other and they are magnetized in an opposite direction to each other as shown in the state B at the room temperature, an interface magnetic domain wall (referred to as interface wall hereinafter) is formed on an interface therebetween because of an exchange coupling force.
The interface wall energy .sigma.w is represented as follows: EQU H.sub.w i =.sigma..sub.w /2M.sub.si h.sub.i ( 1)
Wherein,
H.sub.w i : the effective bias magnetic field received by an ith magnetic layer and caused by another magnetic layer adjacent to the ith magnetic layer,
M.sub.si : the saturation magnetization of the ith magnetic layer,
h.sub.i : the thickness of the ith magnetic layer. EQU .sigma..sub.w =2{(A.sub.1 .times.K.sub.1).sup.1/2 +(A.sub.2 .times.K.sub.2).sup.1/2 } (2)
Wherein,
A.sub.1 : the exchange stiffness constant of the 1st magnetic layer,
A.sub.2 : the exchange stiffness constant of the 2nd magnetic layer,
K.sub.1 : the effective magnetic anisotropy constant of the 1st magnetic layer,
K.sub.2 : the effective magnetic anisotropy constant of the 1st magnetic layer,
As the condition of the overwriting, it is necessary to meet the following conditions so as to prevent the transition from the state A to the state B at room temperature. EQU H.sub.c1 &gt;H.sub.w 1 =.sigma..sub.w /2M.sub.s1 h.sub.1 ( 3)
Wherein,
H.sub.c1 : the coercivity of the 1st magnetic layer,
H.sub.w 1 : the effective bias magnetic field received by the 1st magnetic layer and caused by the adjacent magnetic layer of the 1st magnetic layer,
M.sub.s1 : the saturation magnetization of the 1st magnetic layer,
h.sub.1 : the thickness of the 1st magnetic layer.
Further, to prevent the transition from the state B to the state E at room temperature, it is necessary to meet the following condition. EQU H.sub.c2 &gt;H.sub.w 2 =.sigma..sub.w /2M.sub.s2 h.sub.2 ( 4)
Wherein,
H.sub.c2 : the coercivity of the 2nd magnetic layer,
H.sub.w 2 : the effective bias magnetic field received by the 2nd magnetic layer and caused by an magnetic layer adjacent to the 2nd magnetic layer,
M.sub.s2 : the saturation magnetization of the 2nd magnetic layer,
h.sub.2 : the thickness of the 2nd magnetic layer.
Further, in the state E, to prevent the polarity inversion of the 1st magnetic layer by the initial magnetic field Hini for initialization, it is necessary to meet the following condition. EQU H.sub.c1 .+-.H.sub.107 1 &gt;H.sub.ini ( 5)
Here, as to the positive and negative signs ".+-." of the left side, the positive sign "+" is employed when the 1st magnetic layer is made of a rare earth metal rich (referred to as RE-rich) layer and the 2nd magnetic layer is made of a transition metal rich (referred to as TM-rich) layer, or an opposite case to the above, i.e., in the case of an antiparallel coupling. The negative sign "-" is employed when both the 1st and 2nd magnetic layers are made of the RE-rich layers, or the TM-rich layers, i.e., in the case of parallel coupling.
On the other hand, it is necessary to meet the following condition so as to allow the transition from the state E to the state B. EQU H.sub.c2 +H.sub.w 2 &lt;H.sub.ini ( 6)
Further, to allow the transition from the state C to the state A at about the Curie temperature Tc1 of the 1st magnetic layer, i.e., to allow the magnetization orientation of the 1st magnetic layer to be arranged in the magnetization orientation of the 2nd magnetic layer, it is necessary to meet the following condition. EQU H.sub.w 1 &gt;H.sub.c1 +H.sub.ex ( 7)
Wherein, Hex: the external recording magnetic field.
As seen from the above description, in order to satisfy the formula (3) and (4) at the room temperature, the interface wall energy .sigma..sub.w is desirable to be smaller.
At the temperature where the magnetization orientation of the 1st magnetic layer is arranged in the magnetization direction of the 2nd magnetic layer, the interface wall energy .sigma..sub.w needed to be maintained at a state that the amount of the interface wall energy .sigma..sub.w is not changed or less changed if any, under a condition satisfying the formula (7).
However, it was impossible to realize all the states mentioned above by only controlling the coercivity values, the saturation magnetization values and the thicknesses of the 1st and 2nd magnetic layers.
In the magneto-optic readout method, the magnetic reversal portion corresponding to an information bit, which forms a magnetic domain, is read out based on magnetic Kerr effect. Thus, in order to improve the recording density, it is necessary to decrease a length of a recorded information bit, i.e., to diminish an area of the magnetic domain.
However, as well known, a reproduction resolution of signal is practically determined by a wavelength .lambda. of a light source and a numerical aperture of an objective lens used in an optical pickup system, and its limitation of the readout is determined by 2NA/.lambda..
Thus, in order to increase the recording density, it is considered to decrease the wavelength .lambda. of the light source and/or to employ an objective lens having a high numerical aperture NA to allow a diameter of a laser spot to be diminished. However, a wavelength of the laser beam in a practical use is only 680 nm. Further, when the objective lens having the high numerical aperture NA is employed, a focal length thereof becomes short. This requires a high precision distance control between the lens and an optical disc, resulting in a severe precision of the optical disc in production.
Accordingly, it is impossible to employ such an objective lens having the high numerical aperture NA. The numerical aperture NA of the objective lens to be practically used is at most 0.6. In other words, there is a limitation to improve the recording density by using the wavelength .lambda. of the light source and the numerical aperture NA of the objective lens.
As a countermeasure for solving the problem in improving the recording density restricted by such readout conditions, there are proposed a method for readout signals and a detected medium used for the same, for instance, in Japanese Patent Laid-open Publication 6-150418/1994.
In this readout method, the exchange interaction between a readout layer (first magnetic layer) and a recording layer (second magnetic layer)is utilized in such a manner that the information magnetic domain recorded on the recording layer is copied to the readout layer, and the information is detected from the readout layer.
The structure of such a recording medium is as follows.
The magneto-optic recording medium comprises a transparent substrate, a readout layer (a first magnetic layer) formed on the transparent substrate and a recording layer (a second magnetic layer) formed on the readout layer. The readout layer behaves the in-plane magnetization parallel to the surface thereof at room temperature. However, as the temperature rises, the in-plane magnetization of the readout layer is changed into the perpendicular magnetization. The recording layer has a function to magneto-optically record information thereon. The recording layer is made of a rare earth-transition metal alloy.
The information is recorded on the magneto-optic recording medium as follows.
Under a constant magnetic field applied to the medium to allow the recording layer to be magnetized, a magnetization orientation of the recording layer is reversed by being irradiated with a laser beam in such a manner that the power of the laser beam is selectively set to either a first laser power which is relatively lower laser power, or a second laser power which is relatively higher laser power responsive to the recording signals.
The information recorded on the recording layer is reproduced as follows.
When the laser beam is irradiated on the medium, there is generated a gradient of temperature in the diameter range of the laser beam spot on the readout layer. The readout layer which shows the in-plane magnetization at the room temperature, turns to the perpendicular magnetization in an area corresponding to a high temperature area in the gradient of temperature, and the orientation of the perpendicular magnetization is arranged in the direction parallel to the magnetization of the recording layer. Thereby, the information is read out.
However, the material forming the readout layer in the magneto-optic recording medium mentioned in the foregoing has a wide temperature range for allowing the magnetization transition from the in-plane to the perpendicular magnetization. This wide temperature range causes a problem that it is inadequate to read out information with a high S/N (C/N) from the area information magnetic domain of the recording layer smaller than an area corresponding to the diameter of the laser beam spot irradiated on the readout layer.
On the other hand, according to the present invention, as the material forming the readout layer (the first magnetic layer), there is employed an alloy having a small exchange energy between the rare earth metal and the 3d transition metal. Thereby, it is possible to reduce the temperature range for allowing the magnetization transition from the in-plane to the perpendicular magnetization, resulting in realizing the magnetically induced super resolution readout for the high density recorded information recorded on the area information magnetic domain of the recording layer smaller than the area corresponding to the diameter of the laser beam spot with excellent S/N (C/N) compared with ordinary ones.